The disclosure generally relates to measuring optical properties of an optical multi-mode fiber (MMF) such as bandwidth and, more particularly, relates to methods and configurations for testing an MMF by signal transmission and receipt during processing of the MMF.
Fiber manufacturing typically involves various types of process feedback, many of which involve measurements of the glass and coatings in situ. These measurements include diameter, geometry, and flaw counts. However, these measurements generally do not relate to optical properties of the fiber core, often the most critical for end product attributes (loss, dispersion, bandwidth, etc.). One significant reason why fiber core properties are not measured to provide fiber manufacturing feedback pertains to the difficulty of accessing the fiber core during processing. For instance, the core is being stretched from a large preform to a hair-thin strand, and factors such as high-temperature, high-speed elongation, and rotation/motion of the fiber end preclude easy access for launching and receiving light from the core.
Fiber core attributes are some of the most important attributes for assessing end-product performance, since the core carries optical signals. Moreover, the core region for multi-mode fibers is typically ˜8× larger than the more ubiquitous single mode fibers commonly used in long-distance (many km) optical fiber links, supporting numerous spatial modes (often >100). For this reason, many optical measurement configurations that can be used with single mode fibers encounter additional complexity in implementation and interpretation when applied to multi-mode fibers. For example, meaningful amounts of light must be launched into the larger multi-mode core for adequate signal-to-noise ratios. At the same time, nonlinear effects from high power levels should be avoided, and the proper field localization (i.e., exciting the relevant spatial modes) should be guaranteed in the multi-mode fiber in order to make interpretation of the measurement results accurate.
This goal of accurate measurements is further complicated by the advent of low bend loss fiber profiles, where a refractive index profile feature, such as a trench surrounding the core, dramatically reduces light losses from the core due to bending. This thwarts many known inline coupling techniques since common practices, such as bending the fiber to leak light into a detector, will not be successful.
Bandwidth is one of the most important attributes for MMF products, and is used in the optical fiber industry to set the grade of the MMF product. The bandwidth is directly related to how the refractive index profile of the MMF exists in practice compared to the ideal, optimal profile for a given MMF configuration. The bandwidth exhibited by a given MMF is generally very sensitive to even a slight deviation from the ideal, optimal profile.
Bandwidth is typically measured on cut lengths of optical fiber, after the fiber has been drawn, cut into shipping lengths, and stored on a storage spool. One such method for measuring the bandwidth of an as-produced MMF is to apply a light signal at one end of the MMF, receive the transmitted light signal at the other end, and measure bandwidth through the known, differential mode delay (DMD) measurement technique. The DMD method scans the time delay through a controlled offset launch from a single mode fiber launch condition, and further data processing converts the DMD result into a bandwidth result. The time for each DMD measurement is typically about 5-11 minutes.